The present invention relates to threaded bolts and pins capable of receiving a swaged collar and having impressions in the threads to receive swaged collar material and improve the breakaway torque of the collar from the bolts and pin.
For purposes of nomenclature, threaded male fasteners include threaded pins, bolts, and studs. Pins and bolts have heads that bear on one side of the workpieces, a shank that extends through the workpieces, and threads extending out the other side of the workpieces; studs have threads at both ends. Threaded female fasteners include nuts and collars that have threads that engage the threads of male fasteners and cooperate with the male fasteners in clamping-up workpieces. This specification will generally use "pin" to encompass studs, pins and bolts, and "collar" to encompass both collars and nuts. A thread has a load-bearing flank and a nonload-bearing flank. The load-bearing flank does the work of the thread by loading the corresponding load-bearing flank of the cooperating threaded element. Swagable collars are collars that are plastically deformed into grooves of a male fastener element to make a joint. Preload is the clamp-up force applied to the workpieces by the fasteners, which in turn is applied to the pins by the workpeces.
Threaded pins have been used with swagable collars instead of threaded collars. When used with swagable collars, swaged collar material enters the groove of the male thread and locks the collar and the pin together during the forming of a joint of these two components and two or more workpieces. The use of a threaded pin with a swagable collar increases the flexibility of the pin: it can be used with a swagable collar or a threaded collar.
Threaded pins used with swagable collars differ from lock bolts. Lock bolts have one or more annular grooves that receive swaged material from a collar. These annular grooves are not connected; therefore, even if the collar rotates, it cannot separate from the lock bolt, and the workpieces still experience compressive load from the fastener component. But a lock bolt cannot be used with a threaded collar.
Recognizing the collars swaged onto threaded pins can loosen, some have provided impressions in the thread of a pin to create interference between the two.
An example of these efforts is U.S. Pat. No. 3,421,562 to Orloff et al. The patentees show knurlings in the thread of a pin adjacent the interior end of the thread, the end closest to the head of the pins and next to the pin shank. These knurlings interrupt the crest of the thread and extend to both the load-bearing and nonload-bearing flanks of the thread. Orloff et al. does not extend the knurlings past the zone of the thread that in service occupies the position adjacent one side of the workpieces joined by the pin and a swaged collar. The reason for this, presumably, is that the patentees use a hybrid collar that threads onto the pin, and after applying a clamp-up load to the workpieces the collar is swaged onto the threads in a zone adjacent the workpieces.
National Aerospace Standard 4444 published in 1974 also shows knurls in threads of a threaded pin. These knurls occur at least from the interior end of the thread for several pitch lengths, and can occur all along the length of the thread. The knurls breach the thread crest from the load-bearing to the nonload-bearing flanks.
While effective in increasing breakaway torque between a swaged collar and a threaded pin, the knurls produce substantial friction between the load-bearing flank of the pin and the load-bearing flank of a threaded collar. This friction increases the setting torque required to set the fastener, and when setting torque measures preload reduces the preload on the workpieces. Known preload on workpieces must be reliably applied so that the fatigue life of joints is known and acceptable. Increased frictional loads from the knurls in the thread can reduce preload on the workpieces and reduce fatigue life below acceptable levels.